1. Field of the Invention
The present invention relates generally to orthopedic fixation devices, fasteners, and implants that are used in orthopedic surgery, neurosurgery, plastic surgery, hand surgery, foot and ankle surgery, and Ear-Nose-Throat (ENT) surgery. These devices, such as: pedicle screws, rods, cross-links, plates, set capping screws, spinal fusion interbody spacers for lumbar/thoracic/cervical spine (both stand alone and with associated interbody and plate combination, and corpectomy cages (both stand alone devices and separate corpectomy and plate combinations), among others, are used, for example, to facilitate spine stabilization and healing after spinal fusion surgery; in addition to being used to stabilize other types of orthopedic bony fractures (such as long-bone or facial fractures) in trauma departments. Some of these devices may also be used in plastic surgery and oral/facial surgery, hand and foot surgery, and ear-nose-throat surgery for a variety of pathologies, including mandibular and facial fractures, extremity fractures, oncologic disease of the skeleton and skull, and traumatic disease of the skeleton and skull.
2. Description of Related Art
Millions of people suffer from a variety of musculoskeletal disorders or traumatic occurrences necessitating the use of methods and devices to provide reliable spinal stabilization and facilitate rapid healing. Stabilization may be accomplished with mechanical fasteners, implants, and fixation devices such as pedicle screws, rods, cross-links, plates, vertebra interbody spacers, and corpectomy cages.
Pedicle screw fixation has been shown to be superior to other methods of instrumentation of the lumbar and thoracic spine for spinal fusion and correction of deformity. However, there are many complications associated with placement of screws within the lumbar and thoracic spine, as well as the entire spinal axis. Pedicle perforation is noted to be as high as 40%, which increases likelihood of dural tears, nerve root injuries, paraplegia, and vascular injury.
Non union and pseudoarthrosis rates have been shown to be as high as 10% to 40%. Screw pullout rates are noted to be as high as 5-20%. Infection continues to be a devastating problem post-operatively in patients with spinal instrumentation and fusion.
Surgical techniques for the treatment of spinal injuries or deformities (e.g., scoliosis) are usually aimed at joining together two or more adjacent vertebrae of the spine, through a procedure that is called spinal fusion. A common approach to spinal fusion adopts a fixation system that is anchored to the spine by way of orthopedic screws implanted into the pedicles of two or more adjacent vertebrae. The single screws (i.e., pedicle screws) are connected together by means of rigid or semi-rigid rods, thereby forming a rigid cage that stabilizes and protects the spine. In previous versions, the connecting rod was housed within a transversal hole provided in the pedicle screw head itself. However, due to the irregularity of bone anatomy, it was unlikely that once the screws had been implanted into the spine pedicles that the transverse holes in their heads would be properly aligned for rod insertion. Hence, in order to facilitate the alignment and insertion of the rod, modern pedicle screws are provided with a rotatable, rod-receiving connecting member (connector) that freely rotates and swivels with respect to the screw's shaft.
Screws of this type, named polyaxial screws, comprise a threaded shaft with a hemi-spherical polyaxial. The polyaxial drive end is typically housed inside a mating, hemi-spherical recess (i.e., as a ball & socket joint) provided in the rod-receiving connecting member (rod-connector). A transversal hole, or U-shaped channel, in the rod-connector houses the connecting rod; and a set-screw or threaded-plug insert is provided above the rod, which clamps the rod into a rigid, locked position. In typical polyaxial pedicle screws, such as the example disclosed in U.S. Pat. No. 5,672,176, the locking action of the set-screw determines the locking of both the connecting rod and the bone screw's orientation, since pressure applied by the set-screw is transmitted to both the connecting rod and the screw's hemi-spherical drive head.
These implantable, orthopedic fixation devices are typically made of a rigid material, such as a titanium alloy or stainless steel. While the use of such rigid materials provides sufficient strength and load-carrying capabilities to avoid fractures or breakage, the interface between the metallic device and the surrounding bone is relatively non-flexible and unyielding.
Alternatively, these devices may be formed of semi-rigid materials, such as polymeric materials (e.g., PEEK). While the use of such semi-rigid materials provides a more flexible or yielding interface between the device and the surrounding bone, the strength and structural load carrying capabilities of polymeric bone anchor are generally less than metal alloys.
Bone autograft and allograft and/or autograft materials are a third alternative, and they are commonly used for vertebra interbody fusion spacers, in part, due to their capability for bio-integration at bone-to-bone interfaces. While bone is quite strong in compressive loading, it is relatively weak in tension and shear. For this reason, structural (load-bearing) orthopedic fixation devices and fasteners (e.g., pedicle screws) are not often made of allograft (cadaver bone) and/or autograft bone tissue.
Thus, there remains a need for improved materials for use in orthopedic fixation devices, implants, and fasteners, especially for spinal stabilization, which optimally combine the best properties of all three types of materials described above; and corresponding methods for implementing same.